Circulating fluidized bed technique has for a long time been applied e.g. in calcinators and recently to a larger extent in various reactors such as combustion furnaces and gasifiers. The standard practice for carrying out the separation of the circulating solid material from the flue gases has been to use a conventional cyclone separator having a hopper-shaped bottom. The cylindrical vortex chamber of the cyclone is provided with a gas discharge pipe which guides the gases upwards and the solid material is recycled to the reactor through a stand pipe via a gas trap. The gas trap is employed to prevent reactor gases from flowing into the cyclone through the stand pipe. A mechanical trap is most commonly used as a gas trap or in more advanced applications a fluidized bed of sand in a U-pipe. Especially in high temperature reactors the solid material recycling system becomes complex and expensive. Part of the air required for fluidizing the gas trap flows upwards in the stand pipe which has a detrimental effect on the separation of solid material, in particular on the separation of light and fine particles. Furthermore, a rising gas flow decreases the transport capacity of the stand pipe.
As is known, a substantial underpressure and a high axial flow velocity are created in the center of a conventional cyclone due to which the cyclone tends to suck from the stand pipe. A suction flow generated in this way has usually no tangential velocity, thus almost all the solid material the flow carries with it is transported out through the center pipe of the cyclone. A recycling system provided with a conventional cyclone is therefore very sensitive to the suction flow from the stand pipe and requires a reliable gas trap.
In steam boilers, the use of a conventional cyclone results in a disadvantageous design as a conventional cyclone devices the boiler into a separate combustion chamber and a convection part after the cyclone between which the equipment for recycling the solid material must be installed.
Mechanical gas traps are rapidly worn in particular in hot condition and disturbances in their use are frequent.
International patent application no. WO 85/04117 discloses an apparatus in which solid material is separated in a horizontal cyclone which is disposed on top of the reactor, and to the horizontal vortex chamber of which the flue gases flowing upwards from the reactor are tangentially guided.
In the known apparatus the whole solid material amount contained in the flue gases is transported to the vortex chamber of the separator.